An automobile steering system plays a key role in automobile running safety and driving experience. In a conventional automobile steering system, the transmission ratio is usually a fixed value, that is to say, the transmission ratio of the steering system can not be changed during operation. A contraction exists between light steering effort and flexibility. Namely, in case the torque transmission ratio is increased, the steering effort will become lighter, while the angle transmission ratio will also become larger, indicating the steering is not sensitive. In the conventional automobile steering system, a relatively suitable transmission ratio with fixed value is the result of a compromise. Although there are steering boxbox with variable transmission ratios, e.g., a rack and pinion steering boxbox with variable tooth pitch, rack with recirculation ball and gear sector-type steering boxbox with a variable sector engagement radius, their transmission ratios can be designed to vary in a certain range. However, this certain range is not large, and these steering boxboxes are difficult to process.
The problems mentioned above can be solved by a recent automobile steering system in which the front wheel can steer actively. By adding a set of planet reducing mechanism to a steering column, e.g., in AFS system which has been developed jointly by BMW and ZF Friedrichshafen AG, an input freedom is provided by a planet gear mechanical structure, so as to provide additional steering by an electric servo motor. The Audi Dynamic Steering system has been developed by Audi, which has a variable steering ratio, is capable of varying the steering ratio in a certain range continuously by a harmonic gear set, and can magnify or downscale a steering action of a driver in a certain range. Another active steering system is a drive-by-wire steering system (SWB), in which the driver's steering angle input and the instant automobile status are considered by a controller to determine the output current of the steering motor for driving the front wheels. In the drive-by-wire steering system, the mechanical connection between the steering hand wheel and the steering boxbox is eliminated, reducing the safety. In the AFS system of BMW or the Dynamic Steering system of Audi, while an input freedom is added to the steering system, the mechanical connection between the steering hand wheel and the steering boxbox is still maintained, which not only conforms to the current regulations about automobiles, but also improves reliability of the system.